Known finger joint units for robot hands include those that use worm gears. A worm gear type finger joint unit has a rotary actuator, a worm coaxially fixed to a worm shaft that is rotatably driven by the rotary actuator, a worm wheel that is meshed with the worm, and a joint shaft to which the worm wheel is coaxially fixed. Swing arms (finger main body sections or fingertip sections) are attached to shaft end sections of the joint shaft in a perpendicular state, and the rotation of the joint shaft causes the swing arms to swing.
In a worm gear type finger joint unit of this configuration, when foreign objects enter the bearing sections of the joint shaft and the section where the worm meshes with the worm wheel, this hinders smooth rotation and reduces the longevity of these sections. In cases in which the finger joint unit is used inside a clean room, there is a possibility that a lubricant coated on or supplied to these sections will scatter outward and compromise the cleanliness of the room. To avoid this type of negative result, it is preferable to use a configuration in which the worm, the worm wheel, the joint shaft, and other such components are sealed within a gearbox case.
In the case of a worm gear type finger joint unit, a self-locking mechanism is provided so that the worm cannot be rotated in reverse by the worm wheel. This is useful as a reverse rotation prevention mechanism, but during severe problems such as when the rotary actuator is disabled, the swing arms cannot be moved from the output side, which makes this mechanism inconvenient. Particularly, in the case of a sealed worm gear type finger joint unit, since the worm is housed within the gearbox case, the gearbox case must be disassembled in order to turn the worm manually, which is extremely inconvenient.
If the meshing of the worm gear has backlash, this is amplified by the swing arms attached in a perpendicular state to the joint shaft, and the movement of the swing arms has considerable backlash. In the case of a sealed worm gear type finger joint unit, the worm gear must be incorporated inside a sealed gearbox case. Therefore, it is generally difficult to precisely incorporate the worm gear without any backlash or with minimal backlash.